SOIL ORGANISMS - Soil Ecology - page 276

This is particularly due to an increase of the proportion of Prostigmata whose average
weight is often ca. 0.10 In the soil of a Swedish coniferous forest site, a high
proportion of Prostigmata and juvenile Cryptostigmata Brachythioniidae depressed
the average individual weight to a low 0.16 As a result, biomass estimates do not
show such a clear pattern of thermo-latitudinal variation as population density. It is still
not clear, however, whether these differences reflect existing patterns or simply depend
on the efficiency of the separation of the smallest individuals (Garay, pers.com.).
Tundra communities contain populations whose biomasses range from the lowest
(100 mg f wt) to the highest values, i.e., 1-1.5 Temperate-climate coniferous
forests are in the range 400-1000 mg whereas deciduous forests are split between mor
and moder soils with biomasses greater than 1000 mg and mull soils with lower biomasses
of 100 to 400 mg. In temperate grasslands and in most tropical environments and deserts,
biomasses are low, most often less than 130 mg

4.2.2.2 Community structure

Species Richness
The species richness of acarine communities is often very high. More than a hundred
species have been identified from an Irish grassland (Whelan, 1986) and 72 in a Swedish
grassland (Persson and Lohm, 1977). In temperate forests, species richness may be of
similar order and as many as 52 species of Cryptostigmata have been recorded in a mixed
wood at Foljuif near Paris (Garay, 198la) and 90 species in a chestnut ( Castanea sp.)
wood (Anderson, 1975). In some tundra soils, high species richnesses have been recorded,
e.g., 37 species of Acari at Point Barrow Alaska (McLean, 1980). In more extreme
environments, such as the maritime antarctic island of Signy, however, only one or two
species, with outstanding resistance or tolerance to frost were noted (Block, 1983).
In deserts, water-stress also limits the number of species. Nonetheless, 30 species
have been recorded from the Chihuahuan desert of New Mexico (Kamill et al., 1985).
In sclerophyllous Mediterranean vegetation associations, Asikidis and Stamou (1991)
found 28 species of Cryptostigmata mites.

Taxonomic structure
Cryptostigmata are generally the most abundant group in soils but their relative abundance
varies greatly within single vegetation types. They represent more than 90 % of Acari in
blanket bogs and ferns (Petersen and Luxton, 1982), 17 to 71 % in the tundra soils of
Prudhoe Bay (Alaska), 49-75 % in temperate forests (Wallwork, 1967; Huhta et al., 1986;
Garay, 1981a), 26 to 45 % in temperate and tropical grasslands (Persson and Lohm, 1977;
Athias, 1976), and 16 to 51 % in hot or alpine (Venezuelan paramo) deserts (Petersen and
Luxton, 1982; Garay, 1981b; Kamill et al., 1985).
Mesostigmata are the second most important group with relative abundances ranging
up to a maximum of 20 %. They are more abundant in wooded than non-wooded ecosys-
tems. The density of Astigmata rarely exceeds 1000 although they may achieve
some importance where a deep organic horizon is present. Prostigmata have generally
low, but highly variable abundances with a maximum relative value of 55 % in soils of
the Negev desert (Steinberg and Wallwork, 1985). However, their densities may often